Method for determining the authenticity of ancient artifacts made of obsidian



Nov. 28,

(THICKNESS OF HYDRATED LAYER,MICRONS) (THICKNESS OF HYDRATED LAYER,MICRONS) 1951 I. FRIEDMAN ETAL 3,010,208

METHOD FOR DETERMINING THE AUTHENTICITY OF ANCIENT ARTIFACTS MADE OF OBSIDIAN Filed May 6, 1960 200 FIG. I

c B O l l l l TIME,YEARS 4O FIG. 2

Z I 0 2000 4000 e000 8000 |0,000 TIME,YEARS FIE-3.3

INVENTORS IRVING FRIEDMAN ROBERT L. SMITH BY ATTORNEYS S.a..cs a at Unite ari' i at whine 3,010,208 METHQD FQR DEE'EEM'INEIJG THE AUTHEN- TIClTY 9F ANCENT ARTFACES MABE it? OBSlDIAN Irvin Friedman and Robert L Smith, Bethesda, Mid assignors to the United States of America as repre sented by the Secretary of the interior Filed May 6, 196 3, Ser. No. 27,458 6 Claims. {CL 33-1) (Granted under Title 35', US. Code (1952), see. 256) The invention herein described and claimed may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of royalties thereon or therefor.

The invention relates to a method for determining authenticity of obsidian artifacts, or those made from volcanic glass having in general the same composition as rhyolite. Glass of this type contains between 65 and 7 8 percent silica by weight, approximately 9 to 13 percent alumina, several percent of iron oxide, a total of about 3 to 10 percent of potash and soda, plus lesser amounts of other elements, and contains less than 0.5 percent water by weight. By means of the method of this invention archaeological finds of artifacts ranging in age from approximately 100 to over 30,000 years, may be authenticated.

This method for dating archaeological remains is founded upon the discovery that during long periods of time obsidian absorbs water from the atmosphere forms an ever increasing but extremely thin outer hydrated layer. The depth or thickness of this layer is measurable under a microscope, and when this finding is used in the further steps of the method, there is provided a unique and dependable solution to a fundamental archaeological problem of accurately assigning an age to obsidian artifacts. Since this method is relatively rigorous, and is directly applied to the material of an ar ifact under consideration, it becomes a valuable tecl1- nique for detecting fraudulent obsidian artifacts. it was the heretofore diiliculty in such detection which probably encouraged the manufacture of such fraudulent artifacts.

Research into the source of the water content of obsidian produced a finding that all of the naturally formed surfaces on obsidian under o a chemical and physical change known as hydration. A thin molecular cover of water formed by adsorption adheres to such surfaces, and is slowly absorbed to form a hydrated layer. Viewed in cross section under a microscope the obsidian is re vealed as having a greater optical density in an outer rim thereof due to the water taken up in its surface. It is the difierence between the index of refraction of the denser hydrated rim and that of the rest or" the obsidian the mass which permits a. view of a quite sharp interface between the nonhydrated original obsidian and the hydrated surface. A disclosure of this research may be found in an article by Ross and Smith, entitled Water and Other Volatiles in Volcanic Glasses, published in American Mineralogist, volume 40, pages 1071 to 1689 (1955).

Water contents of hydrated obsidians have been measured at 2 to percent by weight, and that of nonhydrated obsidian at 0.1 to 0.9 percent by weight. The results of continued research provided data to further explain the significance of these differences in water contents. It was demonstrated that the deuterium content of the wa ter in certain types of hydrated volcanic glass, such as periite, is comparable to the deuterium content of the surface water (technically known as meteoric water) in the same region whereas the deuterium content of the water of associated nonhydrated obsidians was in general different from surface water. It was therefore concluded that the nonhyorated obsidian gained its water during its lava or formative stage. Furthermore, that hydrated obsidian was subject to secondary hydration occurring on the surface thereof under normal conditions of atmospheric temperature and pressure on the surface of the earth. Although these findings proved that obsidians can absorb meteoric water to form perlite, and that most perlites contain meteoric water, they did not show whether the hydration of the surfaces of the obsidian was a natural and continuing process, or whether water was taken in during cooling of the lava, or as a result of later hydrothermal alterations.

A disclosure of the continued research may be found in an article by the inventors entitled, The Deuterium Content of Water in Some Volcanic Glasses," published in Geochimica et Cosmochimica Acta, 1958, volume 15, pages 218 to 228, printed in Northern lreland by the Pergamon Press, Ltd.

in order to prove unequivocally the postulate of surficial hydration of obsidians, which provides the basis for the method of this invention, it was necessary to know if the hydration rate for a selected obsidian was rapid enough to account for the geologic occurrences of hydrated portions. To obtain such proof man-made artifacts obsidian were considered as if they were results of experiments of long duration made under different and known conditions. Since artifacts made of obsidian were shaped by chipping the surfaces, a fresh (unhydrated) face would have been exposed by the makers in the process of manufacture. Then these artifacts, flakes, and chips would have been buried as grave or ceremonial offerings, or discarded or lost in the refuse of the habitation sites. The archaeologist, who reconstructs the history of extinct cultures, assigns ages to the objects he finds by means of relative chronology seriation techniques, rates of refuse accumulation, historic records, carbon-14 dates, tree ring dating, calendar correlations of glyphs or ancient carvings, etc. If the age of an obsidian object were known by means of the methods indicated, then the thickness of the hydrated layer, if it existed, could be measured under the microscope, and by correlation with the known age, some idea of the rate of hydration could be established.

A substantial number of archaeological specimens of obsidian having known ages within a span of about thirty thousands of years, were examined for the presence of a hydrated layer. The examination showed hydrated layers of varying thickness, and made evident a significant relationship between the known ages and the thicknesses measured.

For a better understanding of our invention reference may be had to the accompanying drawings in which FIG. 1 is a graph indicating the relationship between time and thickness squared, and FIG. 2 is a graph indicating the difference in the relationships of FIG. 1, as influenced by factors of location or temperature and composition. FIG. 3 is a showing of a photomicrograph of a thin layer of obsidian including a cross section through an exposed surface.

Referring to FlG. 1, the significant relationships previously mentioned are illustrated by means of a graph wherein age in years is related to the thickness of the hydration layer as measured on objects of known age from various sites in temperate zones, in microns squared. The ordinate is sealed in terms of microns squared because many diffusional effects give a straight line when the time is plotted against the square of the diffused distance. More concisely expressed Time=K (tbicknessfi, wherein the value of K is dependent upon such factors as composition and the temperature of the locality in which the obsidian was found. For example. K for rhyolitic obsidian will vary from approximately .04 micron squared per thousands of years for northern Alaska, to 11 microns squared per thousands of years in tropical climates. This expression may be seen to also indicate that the rate of increase of the hydrated layer is inversely proportional to the thickness of the layer (Quadratic Law of Tammann). In the figure, the data from four sites are shown as determining a straight line relationship between age and thickness. Sites A and B indicate samples from Hopewell Mound, Ohio, and Danger Cave, Utah, whose ages were derived by the carbon-14 method. Sites C and D indicate different samples from Shanidar Cave, Iraq, in which artifacts were discovered at various levels or layers in the strata. These distinct cultural layers date back to vastly different times as shown in FIG. 1, indicating that the samples are from approximately 12,000, and 27,000 to 30,000 years ago. Carbon-14 was also used to date samples at sites C and D.

Other factors which might effect the rate of hydration were considered. Relative humidity'of the atmosphere in which the artifact was found was studied early in the research, but was found not an important factor. Obsidian surfaces take in water by chemical adsorption much as silica gel absorbs water. Since the vapor pressure over the obsidian surface is extremely low (in the order of to 3 mm. of Hg), the surface of obsidian has a strong aflinity for water. This process of adsorption continues until the surface is saturated with water molecules. In view of the extremely low rate of diffusion of the water molecules into the obsidian surface, and the relatively low water content thereof, the saturated surface is relatively efiective quite independently of the relative humidity.

The rate of hydration or the diffusion process is dependent on temperature. Artifacts of a known age (dated by carbon-l4), found in permanently frozen ground of the Arctic, hydrated at a much slower rate than did artifacts of the same age from temperate or tropical regions. Hence variations in the thicknesses of hydrated rims of obsidians must be considered within categories determined by the temperature and other climatic heating conditionsof the sites in which the obsidians were found.

Obsidian is a glass composed of a mixture of some eight major components, plus a great number of minor components. Although variations in proportions of the components are small within one province or locality, a comparison of the compositions of obsidians from difierent localities show greater variations in proportions. However, the more typical obsidians such as encountered from localities'as far apart as Egypt and coastal Ecuador, are rhyolitic obsidians previously described. obsidians of difierent compositions may have different rates of hydration and a different temperature coefficient of the rate of hydration. Consequently composition is also a significant factor in the determination of a rate of hydration for a selected obsidian artifact.

In FIG. 2, the graphical representations show a number of time-thickness of hydrated layer relationships each similar to that illustrated by the lines through the site points of FIG. 1. However, each of the lines T, U, V, W, X, Y, Z, of FIG. 2, is indicative of the relationship under different conditions of composition or temperature. Obsidians found in Egypt are represented by line T when the composition was Trachytic, and by line V where the composition was Rhyolitic. A Rhyolitic composition found in coastal Ecuador is represented by line U. Lines W and X, are representative of relationships in two diflierent locations in the Temperate Zone. The relationship as determined by examination of samples found in the Sub- Arctic is represented by line Y, and by samples found in the Arctic is represented by line Z.

Certain obsidian finds showed upon examination abraded surfaces and smooth rounded corners which indicated the effects of erosion. Others showed uneven, finely cracked surfaces which indicated the eifects of burning. Obsidian subjected to such surface destroying influences obviously cannot be used in the practice of the method of this invention.

Based upon the discoveries and research previously described in the present method for determining the authenticity of ancient artifacts made of obsidian was evolved.

The obsidian artifact whose authenticity is being determined is treated preferably as follows:

A. Cutting the thin section A small section of the obsidian artifact, flake or chip is cut out at right angles to the chipped or worked surface. This section is ground down, examined under the microscope, and the thickness of the hydrated layer is then measured. Since the techniques of making the thin section are rather specialized and are also the key to a successful application of the method to any problem of dating, they will be described in detail.

The section is cut with a thin (0.015 thick) continuousrim diamond cut-ofi saw blade. These blades contain a high concentration of fine diamond powder imbedded in a bonded metal rim and not only make a thin, clean cut, but also cause a minimum of chipping of the brittle obsidian. The obsidian specimen is held in the hand and pushed against the water-cooled, 4-inch diameter saw blade revolving at 3000 r.p.m. This type of saw revolving at this speed allows a cut through most small obsidian samples in less than 30 seconds. if the object is a small chip or flake, a complete cross section may be cut off. If it is a projectile point or other large artifact that cannot be defaced by making a cross section, a small V-shaped notch is cut into the edge where it will be least conspicuous and will not destroy the exhibit qualities of the artifact.

The sawed-out section is then enclosed in and cemented with cooked Canada balsam to a square aluminum rod /2 by /2" by 6" long, with the sawed face, which is at right angle to the chipped surface, placed against the rod. The rod and specimen are then held Vertically in a V slotted block-rest above a grinding lap. A suitable grinding means is a 3% diameter steel lap revolving at about 400 r.p.m. continuously charged with a slurry of 303 /2 corundum powder and water. Very slight pressure is applied to the aluminum rod, and grinding is continued until the obsidian section is about to thick. This ground surface is marked with a pencil for identification. Then the rod is warmed to melt the balsam and the obsidian slice is removed. Next, the thin section of obsidian is cemented to a glass microscope slide with cooked Canada balsam, so that the marked face is against the glass slide. The slide with the thin section affixed is now held in the hand and the obsidian is ground again on the same lap to approximately 0.010" thick. The final grinding to 0.00 to 0.003" thickness is done on a glass plate using the same abrasive by rubbing the glass slide and aflixed section gently back and forth or in a circular motion until the desired thickness is reached. The thin section of obsidian is now completed by covering the specimen with cooked Canada balsam and afiixing a cover glass. A more detailed description of apparatus and procedures applicable for cutting and mounting on slides the thin sections of obsidians, is found in an article by Reed and Mergner entitled, Preparation of Rock Thin Sections, published in American Mineralogist, volume 38, pages 1184 to 1203, (1953).

In order for the slide to be usable, a major portion of the hydrated rim must be retained intact and some of this hydrated rim must have been sectioned at right angles to the surface originally chipped by man. The latter condition can be checked quickly under the microscope. As the microscope is adjusted in and out of focus, the inner edge of the hydrated layer (the point where it contacts the nonhydrated obsidian) should not shift its position if the section has been cut perpendicular to the surface. In a thin section cut at an inclination to the surface instead of right angles, this boundary line will appear to shift to the right or to the left of a fixed reference point on the eyepiece scale as the focus of the microscope is moved from the upper surface to the lower surface of the thin section. Usually an area can be found on the slide where there is no inclination of the hydrated rim. If not, one way that this can be remedied is to cut and grind a new thin section of this specimen.

B. Measurement of the hydration rim All measurements may be made with a 12.52: filar micrometer ocular. A 45x objective is used when the hydration layers were more than 2.0 microns thick; a 100x oil immersion apoc'nromatic objective may be used to measure layers thinner than 2.0 microns. With the 45x objective, a single division on the fi-lar micrometer ocular drum corresponds on the petrographic microscope used, to 0.11 micron, while with the 1001: objective one division on the drum equals 0.049 micron.

Identification of the boundary line of the hydration rim takes only a little practice and experience. In FIG. 3, there is illustrated a typical photomicrograph made with ordinary light, of a thin section of an artifact. It shows that the hydrated layer is made plainly evident by a distinct line of demarcation L which is indicative of the interface between the hydrated surface and the nonhydrated original obsidian. The thickness of the hydrated layer is the depth M, measured from the exposed surface N. However, the hydrated rim viewed in ordinary light is more difiicult to observe, although more accurately measured, than when viewed under crossed polarized light. The feature of birefringence of the hydration rim due to the strain in the layer causes the layer of hydration to appear bright under crossed polarized light. Nevertheless gross differences in thickness are easily distinguished.

Other methods for measuring the thickness of the hydrated layer are available, and may be used to advantage. For example, a-measurement of a hydrated rim determined by means of a reflective interferometer avoids the need of cutting into obsidian material. A description of a similar layer thickness measurement by means of the reflective interferometer may be found on page 578 of the text College Physics, by C. E. Mendenhall et al., published by D. C. Heath and Company.

Having determined the thickness of the hydration rim in terms of microns, the numerical value of the latter is squared. Charts containing straight line graphs made in accordance with the teachings disclosed in describing FIGS. 1 and 2, are then prepared, based thereon. Since there is knowledge of the composition of the obsidian artifact being tested, of the locality where the artifact was discovered, and of the temperature conditions related to the locality, the graph applicable is therefore identifiable. Using the identified graph, the previously obtained thickness of the hydration layer in microns squared is found as a point on the ordinate scale. In the usual manner, a trace across from this point parallel to the abscissa is made and the latter is read where the trace intersects the graph line, to give for the obsidian artifact under consideration, an authentic age in years.

We claim:

1. A method for determining the authenticity and approximate age of an obsidian artifact, wherein employing a chart graphically presenting a continuous range of data, based upon a plurality of data samples, each relating to a thickness of a hydrated layer measured from the surface of a selected authenticated obsidian, in terms of microns squared, to time in terms of years, comprising, preparing a sample of said artifact in the form of a thin crosssectional portion having as at least one boundary an edge from a surface of the artifact subject to hydration, viewing the sample under a microscope whereby the presence of a hydrated layer including said edge may be detected, measuring its thickness in microns, and applying the square of the numerical value of microns to the prepared chart, whereby a time reading at one point in the said range corresponding to the square value of thickness reveals an authentic age for the said artifact.

2. A method for determining the authenticity and approximate age of an obsidian artifact including the step of preparing a thin layer sample of the artifact in the form of a cross section wherein is visible an edge from a surface subject to hydration, and a portion of the obsidian starting thereat, a further step of measuring on said sample the thickness in microns of a hydrated rim starting at said edge and extending into said portion, and a final step of applying the square of the numerical value of said thickness to a representation on a graph relating thickness in microns squared to age in years, whereby an authentic age for the obsidian artifact is determined.

3. In a method for determining the authenticity of an obsidian artifact comprising the use of data on graphs representing relationships between a factor of thickness of a hydrated layer on the surface of obsidians to a known age factor predetermined for the obsidians, a first step of preparing a specimen of the obsidian artifact on which is visible a cross section thereof including a surface edge and a hydrated layer extending from said edge and into the portion of the obsidian starting thereat, a second step of measuring a thickness defined by the edge and the depth of a hydrated layer starting from the surface, and a final step of fitting to the data on one of the graphs the numerical value of the thickness squared, whereby an authentic age for the obsidian artifact is determined.

4. A method for determining the authenticity of an obsidian artifact comprising the steps of preparing a thin section sample of the artifact wherein is visible a cross sectional portion from an edge of an exposed surface of the artifact which is subject to hydration, measuring the thickness of a hydrated layer starting from the edge and which extends into said portion, and applying the square of the thickness to a graph relating thickness in microns squared to age in years whereby the approximate age and authenticity of said artifact are determined.

5. In a method for determining the authenticity of an obsidian artifact comprising the use of data on graphs representing relationships between a factor of thickness of a hydrated layer on the surface of obsidians to a known age factor predetermined for the obsidians, the steps of measuring the thickness of a hydrated layer from an exposed surface of the artifact, and of fitting to the data on one of the graphs the numerical value of the thickness squared whereby an authentic age for the artifact is determined.

6. A method for determining the authenticity and approximate age of an obsidian artifact which comprises: 1) the step of preparing a thin cross-sectional layer sample of the artifact, said sample including a surface edge and a hydrated layer extending inwardly from said edge, and (2) measuring the thickness of said hydrated layer in microns, whereby on applying the square of the thickness to a graph relating thickness in microns squared to age, the approximate age and the authenticity of said artifact are determined.

References Cited in the file of this patent A New Dating Method Using Obsidian: part 'I, The Development of the Method, by Robert'L. Smith and Irving Friedman, pp. 476-522, American Antiquity, vol. 25, No. 4, April 1960.

A New Dating Method Using Obsidian: part II, An Archaeological Evaluation of the Method of Clifford Evans and Betty J. Meggers, pp. 523-537, American Antiquity, vol. 25, No. 4, April 1960. 

